It is disclosed in U.S. Pat. No. 6,932,774 (JP 2004-24684A) that changes in loads applied to a sleeper's bed are sensed by a sensor sheet having multiple pressure sensitive elements inserted in a lower portion of the bed. Furthermore, a method is disclosed which produces a respiratory body movement signal as a respiratory signal being a signal component of a frequency band corresponding to the number of sleeper's respirations from the load signal, and determines apnea states including an obstructive apnea state from a change pattern of amplitudes of the respiratory signal. In the obstructive apnea state in which a throat is blocked because of a throat muscle slacked by sleeping, and oxygen is not supplied to lungs, the sleeper performs respiratory operations, but oxygen is not supplied to the lungs because the throat is blocked. As a result, the oxygen saturation in blood decreases, and the sleeper temporarily falls into an awakened state and performs a very deep respiratory operation (labored respiration). As a result, the amplitudes of the respiratory signal increase suddenly. Therefore, by detecting the sudden or harp increase in amplitudes, an apnea state can be determined.
The sleep apnea syndrome includes obstructive apnea or central apnea that little oxygen is supplied to lungs, and hypopnea that oxygen supply is insufficient because a sleeper's respiratory tract is narrowed. Furthermore, since the frequency of the respiratory signal when the amplitudes increase becomes higher in comparison with the frequency of the respiratory signal when the amplitudes decrease, the amplitude decrease state of the respiratory signal is determined to be a sleeper's apnea state or hypopnea state.
A respiratory signal obtained from a load signal may include various noises. An example is periodic limb movements (PLM). Therefore, the average of multiple amplitudes in a respiratory signal is calculated, and a change in amplitudes of the respiratory signal is determined from a difference between the averages, thereby eliminating single-shot noise components.
To determine the severity of the sleep apnea syndrome, Apnea Hypopnea Index (AHI) is used. The AHI indicates the total number of apneas and hypopneas per hour of sleeping. To measure the AHI, polysomnography (PSG) is used. The PSG records various physiological changes in electroencephalogram, electrooculogram, electrocardiogram, muscular movements, and the like throughout sleeping of one night, and analyzes the results, thereby evaluating the type and the severity of the sleep apnea syndrome. The examination by the PSG is considered to have high accuracy because it records various physiological changes. However, since the PSG requires that various sensors must be attached to a patient, a significant burden is placed on the sleeping patient. It is thus difficult to detect situations during natural sleeping. Furthermore, much time is required to analyze examination results, and high costs are required for the analysis.
In this regard, the above method of obtaining a respiratory signal by a load signal is simple, places little burden on a sleeping patient, enables detection of situations during natural sleeping, makes the analysis of examination result relatively easy, and is therefore inexpensive. Therefore, the apparatus and method of analyzing a respiratory signal are suitable as a measuring apparatus or a process for screening to determine whether a patient requires measurements by the PSG. Furthermore, application to an apparatus and a process of determining the severity of the sleep apnea syndrome is under consideration. However, to achieve the object, a further increase in the measurement accuracy of a measuring apparatus for screening is demanded. Furthermore, hereinafter, to differentiate from the AHI indicating measurement results of the PSG, an apnea hypopnea index measured by the present apparatus or method is described as pAHI. The pAHI indicates the number of apnea states or hypopnea states per hour.